The field of this invention is industrial trucks powered by electric motors, and particularly, the electric steer systems for such trucks.
Industrial trucks such as lift trucks include a mast which is mounted to a tractor. The tractor includes a set of wheels which facilitate truck movement within a factory, a warehouse or the like. At least one wheel is driven by an electric motor to move the industrial truck, and the same or other wheel(s) are steerable in response to commands produced by manual operation of a steering tiller or steering wheel, or automatic operation of a wire guidance system. The steer unit includes an amplifier which provides current to an electric steering motor that drives the steerable wheel(s) to a commanded orientation as described, for example, in U.S. Pat. No. 5,778,327. A feedback device provides a signal indicative of steer unit orientation and this may be used to indicate steer wheel orientation to the truck operator as described in U.S. Pat. No. 5,680,099.
The proper operation of the steer unit is important whenever the industrial truck is in motion. Conditions that may affect the functionality of electric steer units include shorted, open or disconnected wires, shorted or open motors, and damaged circuitry. As a consequence, it is common practice to periodically check the integrity of the steer system and bring the truck to a halt, or disable operation of the truck if a problem is detected.
One method for checking the integrity of the steer system is to pulse the steer motor at a set rate and pulse duration. If there is no resulting current flow through the steer motor, a fault is declared. One limitation of this method is that it only checks for open circuit failures, not short circuit conditions. Another limitation is that it only partially checks steer unit integrity by sensing current flow in one direction through the steer motor.
The present invention is a method and apparatus for checking the integrity of an electric steering system on an industrial truck. More particularly, steering commands of alternating polarity are applied to the steer unit during truck operation, the resulting current flow through the steer motor is measured, and a fault is indicated if the measured current is below a preset amount (open circuit condition) or above a second preset amount (short circuit condition). These commands do not produce any net change in truck direction but they result in measurable current flow alternately in each direction through the steer motor.
A general object of the invention is to more fully test the steer unit integrity on an industrial truck. By measuring current flow through the steer motor and comparing it with both upper and lower preset limits, both open, and short circuit conditions are detected. By alternating the polarity of the voltage pulses, a more complete check of system integrity is made without affecting steering orientation.
One of the advantages of this invention is that the integrity check of the steer system is accomplished in a background mode, when there is no overt request for a change in steered direction. In normal operation, a lift truck may travel a considerable distance/time in the absence of any steering request from the operator or wire guidance system. The advantage of this approach is that the system is constantly monitored, and is capable of detecting faults in the steer system prior to the need for steering.
The foregoing and other objects and advantages of the invention will appear from the following description. In the description, reference is made to the accompanying drawings which form a part hereof, and in which there is shown by way of illustration a preferred embodiment of the invention. Such embodiment does not necessarily represent the full scope of the invention, however, and reference is made therefore to the claims herein for interpreting the scope of the invention.